Yes, you can. But the usual setup is to have a file system root that is nothing but subvolumes, which you can then use and mount basically as if they were independent partitions. But when you don't create a root subvolume for your system root first, you install the system directly on the file system root alongside created subvolumes. This tends to get messy as strictly speaking the file system root is a subvolume, too. So now you have that with your system installed and all other subvolumes nested inside it.